As a subject for biometrics authentication, there is used inherent structure which exists inside a living body such as a blood vessel. Not being stolen directly from a living body and it is difficult for a third party to simulate a registrant as compared with inherent structure which exists on the surface of a living body such as a fingerprint, inherent structure which exists inside a living body is becoming remarkable in enhancing security.
Conventionally, as an authentication apparatus of this type, there is proposed a technique that picks up an image of a blood vessel by utilizing a phenomenon that a light of near infrared ray region is conspicuously absorbed by deoxidized hemoglobin (venous blood) or oxidized hemoglobin (arterial blood) flowing through a blood vessel, and judges whether or not a certain user is a qualified user based on a blood vessel forming pattern in a blood vessel image obtained as the image pickup result (for example, refer to Patent Document 1).
This authentication apparatus emits a near infrared light, whose intensity is higher than that of a reflected light (an ordinary light in the atmosphere of a visible light etc.) coming from a living body, to a finger from the palmar surface of finger, and guides a near infrared light, which is obtained when the near infrared light is absorbed by hemoglobin in the blood vessel structure inside the finger and scattered by other structures other than the blood vessel structure, to a CCD (Charge Coupled Device) through a macrolens that transmits a near infrared light.
Then, with respect to electric charge amount per unit time period charged by the CCD when the near infrared light undergoes photoelectric conversion, the authentication apparatus adjusts the CCD such that the image pickup sensitivity of the CCD for the near infrared light becomes higher than that for an ordinary light to generate a blood vessel image signal, then the authentication apparatus judges whether or not a certain user is a qualified user based on a blood vessel forming pattern in the blood vessel image signal.
Accordingly, the authentication apparatus can reduce the noise component based on an ordinary light in the atmosphere of a visible light, etc. and improve the image quality of a blood vessel image, which consequently can improve the authentication precision.
Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2004-135609
When a finger is out of alignment in position at the time of picking up an image, even if the image quality of a blood vessel image is favorable, thus configured authentication apparatus undesirably brings about degradation of authentication precision.
As a countermeasure to prevent degradation of authentication precision resulting from the positional misalignment, there may be proposed a method to detect positional misalignment of a blood vessel image from a reference image. On the other hand, since a blood vessel image is an image inside a living body, the detection precision becomes high when detecting positional misalignment using information on the surface of a living body.
In case of applying this detection method to the authentication apparatus, the authentication apparatus has to pick up both an image of the surface of a living body and an image of a blood vessel inside the living body, which undesirably requires processing burden by just that much and lowers immediacy. In this case, when it is assumed that the focal depth is adjusted for both the surface of a living body and a blood vessel inside the living body, control processing for an optical system for adjusting the focal depth becomes complicated due to the difference of depth of a blood vessel from the surface of skin depending on the level of fat of a living body, which undesirably further lowers immediacy.